Electron-bombardment ion beam sources have been employed in sputter machining to selectively remove material from non-protected portions of a target substrate and in sputter deposition wherein portions of a substrate are masked to selectively deposit sputter material by ion beam bombardment in accordance with a predetermined pattern. Further, such ion beam sources have been employed in the implantation or doping of ions into a semiconductor material. In the case of the latter, higher ion energy is a requisite if useful penetration depths are achieved for the doping material.
Basically, all electron-bombardment ion beam sources require a chamber into which an ionizable material (generally in vapor form) such as argon, arsenic etc. is introduced. The chamber bears both an anode and a cathode, with the anode attracting high velocity electrons from the cathode. The impingement of electrons upon atoms (molecules) of the introduced vapor results in the ionization of the atoms (molecules). Typically at one end of the chamber, there is provided an apertured electrode followed by an apertured extraction electrode and a potential is impressed upon the latter electrode which accelerates the ions out of the chamber through the apertures in both electrodes.
Further, the interior of the chamber is subjected to a magnetic field to effect gyration of the electrons in their travel towards the anode, thus greatly increasing the chance of an ionizing collision between any given electron and one of the source material atoms. This results in an increased efficiency in ionization.
Typically, ion sources for isotope separators and implantation systems have used solenoid magnetic fields for increasing the plasma density and gas efficiency by increasing the path length of the ionizing electrons between the cathode and anode. However, as this solenoid field is increased, the plasma constricts, becomes noisy, and the helical instability limits the regime of effectiveness of the magnetic field.
Recent efforts have shown that a quiescent plasma may be obtained by using a multipole containment of the ionizing electrons. U.S. Pat. No. 3,969,646 to Reader et al., issued July 13, 1976, teaches the use of a multipole configuration wherein a plurality of successively-spaced segments of electrically-conductive magnetic material are distributed within the chamber, the segments being interconnected with a potential impressing means so that the segments collectively constitute the anode. Further, individually adjacent segments are respectively polarized oppositely in a magnetic sense so that segments collectively establish the magnetic field. In one specific structural assembly, each of the anode segments is composed of a strip of magnetizable material and each successive pair of such strips are spaced apart by respective individual magnets.
Ion sources used for high current in implantation and isotope separation generally operate at higher current densities, higher source temperatures (.about.1000.degree. C.) and higher extraction voltages than existing steady state sources using multipole configurations. For operation at high voltage, a source should operate at high efficiency to reduce the pressure in the extraction region which may lead to voltage breakdown. In particular, isotope separation requires high efficiency to minimize material losses to the vacuum system. Higher efficiencies are usually obtained by operation at higher plasma densities and hence higher source temperatures. Higher plasma densities can be achieved more easily in a small source geometry for a given cathode emission. Furthermore, a small source geometry is desired to achieve adequate high voltage isolation without extensive consumption of space.
Such multipole sources as found in the prior art are generally large area multi-aperture sources in which the ratio of the low (&lt;100 Gauss) intensity magnetic field region to high intensity magnetic field region is reasonably large. The region of low magnetic field intensity corresponds to the region of quiescent plasma formation during source operation. When constrained to small volumes, commercially available sources do not assume multipole configurations due to the difficulty in achieving a reasonable region of low magnetic field intensity and also in extracting an ion beam in the presence of the high magnetic fields desired at the walls. Furthermore, the high source temperatures resulting from operation at high plasma densities are not compatible with most permanent magnets which tend to lose field strength when heated.
Nevertheless, applicants have determined that a multipole source configuration has certain properties which are advantageous for the high voltage, high efficiency operation of ion implantation or isotope separation sources. Multipole configurations reduce the ionizing electron losses which improves the source efficiency for a given cathode emission level, which would lead to longer cathode lifetimes. Also, since it is extracted from a quiescent plasma, the ion beam from a multipole source will be less noisy and less likely to strike electrodes, producing secondary electrons which can lead to arcing. Furthermore, it has been determined that the presence of magnetic fields in the extraction gap (such as occurs in standard solenoidal field configurations) leads to increased probability of ionization of residual gases by secondary electrons, which enhances conditions for voltage breakdown. The multipole configuration minimizes the level of magnetic fields in the extraction region.
Typically, multipole sources are large broad beam, multi-aperture sources. However, in ion implantation and isotope separation, one wants to achieve both high beam current density and reasonable (50 hours or more) filament lifetime. Of necessity, such a source is one in which the plasma volume is considerably smaller than prior art multipole sources and one which operates in the 1000.degree. C. temperature range.
Accordingly, it is an object of the present invention to provide an ion implantation and isotope separation ion source which will operate at high temperature and high plasma density while maintaining 50 hours or more filament lifetime.
It is a further object of the invention to provide a quiescent plasma ion source compatable with high extraction voltages (80 kilovolts or more).